Integrated optics breakthrough! First verification of high-bandwidth, high-speed photodetectors on lithium niobate waveguide chips
The 21st century is the era of high-speed communication and big data cloud computing, and optoelectronic integrated circuits are expected to be high-performance, low-cost, mass-producible solutions for communication/sensing and quantum computing applications. With many unique material properties such as large electro-optical coefficient, second-order nonlinear polarization rate and wide transparent optical window from visible to infrared wavelengths, lithium niobate is expected to be one of the most suitable material platforms for optoelectronic integrated circuits.
High-bandwidth electro-optical modulators have been developed as a chip-integrated thin-film lithium niobate device and are compatible with the CMOS operating voltage range. Low-loss lasers have also been integrated on thin-film lithium niobate chips in the near future. Thus, except for photodiodes, the basic devices required for optoelectronic integrated circuits have been realized on the thin-film lithium niobate platform and have demonstrated high performance.
As an important part of optical communication networks and microwave photonic integrated systems, photodetectors can convert modulated high-speed optical signals into photocurrents. Typically, the optical signal in a photonic integrated circuit is coupled into a separate photodetector through an optical fiber, but this approach introduces excess optical loss and does not meet the need to implement the photonic integrated circuit on a chip.
To address these issues, the research team of Prof. Andreas Beling and Prof. Xiangwen Guo at the University of Virginia, in collaboration with Dr. Mian Zhang, CEO of Hyperlight Corporation (a startup company founded at Harvard in 2018), and Prof. Marko Lončar at Harvard University have experimentally validated for the first time a high-bandwidth, high-speed photodetector integrated on a lithium niobate waveguide. The related research results were published in Photonics Research 2022, No. 6.
The approach reported in this article performs a series of optimizations on indium phosphide-based single-row carrier photodetectors for higher bandwidth and higher photoconversion efficiency, and enables the integration of lithium niobate and indium phosphide heterostructures through wafer bonding.
As shown in Figure 1, the photodetector is integrated on top of a thin-film lithium niobate waveguide from which light is coupled into the absorber layer and converted into photocurrent. In the experiment, the lens fiber is used to couple the light at 1550 nm wavelength and the photocurrent is extracted through the high frequency probe.
Figure 1 (a) Schematic diagram of the photodetector integrated on a thin-film lithium niobate waveguide; (b) optical microscopy picture of the photodetector: optical fiber (left) and high-speed probe (right); (c) optical microscopy picture of the complete chip with integrated electro-optical modulator and photodetector
Experimental results show that the photodetector achieves a record 80 GHz/3 dB bandwidth (Fig. 2a) and a high responsiveness of 0.6 A/W. The digital data detection performance of the photodetector is characterized by an eye diagram (Figure 2b): non-zeroing switch keying data patterns are generated at 40 Gbit/s and are fed into a thin-film lithium niobate waveguide. By comparison with commercial photodetectors, Figure 2b shows a clear and open eye diagram of the integrated photodetector. It is worth mentioning that the integrated photodetector has the ability to handle enough photocurrent and provide a high signal-to-noise ratio for the oscilloscope, so no amplifier is needed to boost the electrical signal in the measurement. The clear eye diagram demonstrates the high bandwidth performance of the integrated photodetector and demonstrates its capability for digital communication applications.
Figure 2 (a) 3 dB bandwidth measurement results (small area photodetector up to 80 GHz); (b) experimental structure of eye diagram measurements (top of the figure) for a commercial photodetector (left of the figure) and an integrated photodetector on thin film lithium niobate (right of the figure)
According to Prof. Andreas Beling, "The experimental results show that heterogeneously integrated single-row carrier photodetectors on indium phosphide substrates are strong candidates for optical signal detection in optoelectronic integrated circuit devices."
Author Bio.
Andreas Beling
University of Virginia, USA
Research interests: optoelectronic devices, millimeter wave and terahertz electronics, wireless optical communication systems
Andreas Beling, Professor, School of Electrical and Computer Engineering, University of Virginia, U.S.A. He received his master's degree from the University of Bonn, Germany in 2000 and his Ph.D. in electrical engineering from the Technical University of Berlin, Germany in 2006. from 2006 to 2008, he worked as a research assistant at the University of Virginia, U.S.A., and as a project manager for coherent receivers for fiber optic communication systems. He has been an assistant professor at the institute since 2013 and has published more than 130 papers in scientific journals, authored 3 book chapters and 3 patents. He has served as a member of the American Optical Fiber Communication (OFC) Conference (2010-2012), International Conference on Indium Phosphide and Related Materials (2014), International Conference on Microwave Photonics (2015, 2016) and International Program Committee on Integrated Photonics Technologies (2016). He is a member of the Optical Society of America (Optica) and the Institute of Electrical and Electronics Engineers (IEEE), and has been an Associate Editor of the Journal of Lightwave Technology since 2014.
https://www.researching.cn/articles/OJ716567b4d045fd38
